This invention relates to a process for producing oxidized fibers and carbon fibers and, more particularly, to a process for producing carbon fibers with excellent mechanical properties from oxidized fibers prepared by oxidizing precursor fibers in a shorter time than in a conventional precursor-oxidizing step.
For the industrial production of carbonizable oxidized fibers and carbon fibers, there has been widely employed a process of heating raw fibers or precursor fibers composed of, for example, acrylic fibers, tar pitch or petroleum fibers, rayon fibers or polyvinyl alcohol fibers in an oxidative atmosphere at 200.degree. to 400.degree. C. and further carbonizing the resulting oxidized fibers in an inert atmosphere at least at 800.degree. C. In this process, the step of converting the precursor fibers to oxidized fibers requires an extremely long time. Therefore, an attempt of shortening the time for improving productivity has been made by raising the temperature of the oxidative atmosphere or by rapidly raising the temperature to shorten the time. However, it has resulted in formation of a large amount of pitch or tarry product from the precursor or in adhesion of the single filaments, thus causing deterioration of quality of oxidized fibers. As a result, carbon fibers with excellent mechanical properties have been obtained with difficulty from such deteriorated oxidized fibers. In addition, exothermic heat produced by the precursor is accumulated in the precursor, and hence a so-called run-away reaction takes place and, in the worst case, the precursor fibers can be broken or burnt. This tendency becomes serious in the case the precursor is baked in the form of a thick bundle of more than several-thousand deniers for raising productivity.